Once upon a time, virtual was installed under full virtualization (KVM) -mode. Network wasn’t bridged, it was routed.
And then the problem: IPv6 -traffic flowed nicely inbound, but outbound was capped to about 128 kbps.
Troubleshooting: after googling around with ipv6, kvm and debian we came around to this, Debian bug report about GSO Ipv6 issues under KVM. It’s supposed to be fixed in 2.6.32-5-amd64 (2.6.32-39) -package…well, upgrading didn’t work for us, propably another (similar kind of) bug or something needs to be done at host-machine.
Workaround: disabling virtio_net’s gso worked for us:
ifdown eth0; modprobe -r virtio_net; modprobe virtio_net gso=0; ifup eth0